In the field of steel sheet for automobiles, there is an expanding application of high-strength steel sheets that have high tensile strengths so as to establish the compatibility between fuel efficiency and crash safety, backed by increasing stringencies of recent environmental regulations and crash safety standards. However, with an increase in strength, the press formability of a steel sheet decreases, and it becomes difficult to produce a product having a complex shape. Specifically, there arises a problem of a rupture of a high worked region owing to a decrease in ductility of a steel sheet with an increase in strength. In addition, there also arises a problem of spring back and side wall curl that occur owing to residual stress after the work, which degrades dimensional accuracy. Therefore, it is not easy to press-form a high-strength steel sheet, in particular a steel sheet having a tensile strength of 780 MPa or higher into a product having a complex shape. Note that, in place of the press forming, roll forming facilitates work of a high-strength steel sheet. However, the application of the roll forming is limited to components having uniform cross sections in a longitudinal direction.
For example, as disclosed in Patent Document 1, a hot stamping technique has been employed in recent years as a technique to perform press forming on a material having difficulty in forming such as a high-strength steel sheet. The hot stamping technique refers to a hot forming technique in which a material to be subjected to forming is heated before performing forming. In this technique, since a material is heated before forming, the steel material is softened and has a good formability. This allows even a high-strength steel material to be formed into a complex shape with high accuracy. In addition, the steel material after the forming has a sufficient strength, because quenching is performed with a pressing die simultaneously with the forming.
In addition, Patent Document 2 discloses a hot forming member that has both a stable strength and toughness, and discloses a hot forming method for fabricating the hot forming member. Patent Document 3 discloses a hot-rolled steel sheet and a cold-rolled steel sheet that are excellent in formability and hardenability, the hot-rolled steel sheet and the cold-rolled steel sheet having good formabilities in pressing, bending, roll forming, and the like, and can be given high tensile strengths after quenching. Patent Document 4 discloses a technique the objective of which is to obtain an ultrahigh strength steel sheet that establishes the compatibility between strength and formability.
Moreover, Patent Document 5 discloses a steel grade of a high strength steel material that is highly strengthened and has both a high yield ratio and a high strength, the high strength steel material allowing the production of different materials having various strength levels even from the same steel grade, and discloses a method for producing the steel grade. Patent Document 6 discloses a method for producing a steel pipe the objective of which is to obtain a thin-wall high-strength welded steel pipe that is excellent in formability and in torsional fatigue resistance after cross-section forming. Patent Document 7 discloses a hot pressing device for heating and forming a metal sheet material, the hot pressing device being capable of promoting the cooling of a die and pressed body to obtain a pressed product excellent in strength and dimensional accuracy, in a short time period, and discloses a hot pressing method.